A method and an apparatus for cooking and spray-drying starch are provided herein. The starch is uniformly cooked to gelatinization in an atomized state by means of an apparatus comprising a two-fluid, internal-mix, spray-drying nozzle, coupled to a means for drying the cooked, atomized starch.
Problems associated with drying gelatinized starch are well-known in the art. Drum-drying processes expose casted sheets of gelatinized starch dispersions to open air for relatively long periods, permitting contamination; require scraping from a drum, flaking and milling, permitting metal particles to enter the starch; and generally place high demands upon manufacturing space, time and energy. Due to the high viscosity of fully gelatinized (cooked and hydrated) starch dispersions, spray-drying has been used only for very low concentration gelatinized starch dispersions, or for converted starches which have been severely degraded by conversion processes so that the viscosity of the gelatinized starch dispersion is greatly reduced, or for starch slurries in their granular, unhydrated or ungelatinized form.
In addition to the inefficiencies of conventional spray-drying and drum-drying processes, in many food applications the organoleptic quality of gelatinized starch dispersions prepared from granular starches is superior to that of pregelatinized starch dispersions prepared from reconstituted, conventionally-dried cold-water-swelling ("pregelatinized") starches. This quality difference is attributed to the tendency of the pregelatinized starch granules to swell to bursting, releasing their amylose and amylopectin contents, and irrevoccably losing their granular structure, under the mechanical shear and thermal conditions of drum-drying and conventional spray-drying. Pregelatinized starches which are dried under these severe processing conditions have large amounts of granule destruction and may form undesirable pasty, grainy or mealy textures upon reconstitution in aqueous dispersions. Furthermore, absent chemical crosslinking and derivatization, conventionally dried starches, particularly drum-dried starches, typically do not sustain a desirable viscosity during prolonged heating and other processing conditions which are encountered in food applications.
Many conventional spray-drying processes have been disclosed. The particular textural and viscosity problems associated with spray-drying high viscosity hydrated or gelatinized starch are addressed in U.S. Pat. No. 4,280,851, issused Jul. 28, 1981 to Pitchon, et al., which provides a dual-atomization process for cooking or gelatinizing materials. In a first atomization step, this process employs a nozzle for high pressure spraying (atomizing) of the material to be cooked (e.g., granular starch slurry) through a small spinner/orifice into a precisely scaled, enclosed chamber. There the material is cooked by injection of a heating medium. In a second atomization step, the heating medium atomizes the cooked starch dispersion through a vent aperture at the bottom of the chamber and forces the cooked material into a dryer. An apparatus for carrying out this dual-atomization process is disclosed in U.S. Pat. No. 4,600,472, issued Jul. 15, 1986 to Pitchon, et al.
The Pitchon process and apparatus disadvantageously require a specially machined nozzle and chamber, which nozzle contains a starch slurry feed port with small orifices (0.016-0.042 in.) and spinners to achieve the first atomization step. The orifices plug easily and erode quickly under the high pressures (5000-7000 psig) needed to atomize the starch slurry through the feed port orifices and into the enclosed chamber. Additionally, the nozzle must be coupled to a high pressure pump to maintain the pressure needed for the first atomization step. These pumps, like the Pitchon nozzle, require frequent maintenance which limits the efficiency of the Pitchon spray-drying process.
Similar disadvantages are inherent in the starch spray-drying process disclosed in U.S. Pat. No. 4,847,371 to Schara, et al. A dual-atomization process and an apparatus similar to those of the Pitchon patents are employed by Schara, et al. to prepare certain pregelatinized dent cornstarch derivatives. Schara, et al. teach that the Pitchon process and apparatus are ill-suited for cooking and spray-drying corn starch, whereas the Schara, et al. process and apparatus are ill-suited for cooking and spray-drying tapioca starch.
It has now been discovered that a variety of high quality pregelatinized spray-dried starches may be efficiently produced without first atomizing the granular starch slurry. This is accomplished by employing modified, commercially-available standardized spray-drying nozzles and a relatively low pressure granular starch slurry feed in a single atomization step process.